1. Field of the Invention
The present invention relates to a transmitting/receiving apparatus for performing transmission and reception by using a plurality of spreading codes and more particularly, to a transmitting/receiving apparatus which can increase and decrease the number of spreading codes in accordance with a grade of link quality.
2. Description of the Related Art
A conventional example of a transmitting/receiving apparatus for performing transmission and reception by using a plurality of spreading codes is shown in block diagram form in FIG. 1. When the transmitting/receiving apparatus is installed in a base station which can afford to use 20 spreading codes for reverse and forward links, respectively, at a time, it operates as will be described below. Since this base station can use 20 spreading codes at a time, it can accommodate 20 mobile stations using a single spreading code.
Firstly, the operation of a receiver of the transmitting/receiving apparatus will be described. By transferring a switch 202 to a down-converter 204, a receiving signal 203 received by an antenna 201 is inputted to the down-converter 204. In the down-converter 204, a carrier frequency of the receiving signal 203 is down-converted into an IF frequency, so that the receiving signal 203 is converted into an IF signal 205. In a quadrature detector 206, the IF signal 205 from the down-converter 204 is quadrature-detected, so that the IF signal 205 is converted into an in-phase signal (I-ch) 207 and a quadrature signal (Q-ch) 208. The in-phase signal 207 and quadrature signal 208 are inputted to analog/digital converters (A/D converters) 213 and 214 via low-pass filters (LPF's) 209 and 210 so as to be converted into a digital in-phase signal 215 and a digital quadrature signal 216, respectively. The LPF's 209 and 210 are used to suppress frequency components in the in-phase and quadrature signals 207 and 208 which are 1/2 or more of a sampling frequency in the A/D converters 213 nd 214 and to shape waveforms of the in-phase and quadrature signals 207 and 208. The digital in-phase signal 215 and digital quadrature signal 216 delivered out of the A/D converters 213 and 214 are inputted to 20 demodulating units 217.sub.1 to 217.sub.20. First to twentieth codes 227, which are identical to the 20 spreading codes used during transmission, are generated by a reception code generator 226 and inputted one by one to the demodulating units 217.sub.1 to 217.sub.20. The demodulating unit 217.sub.1 includes despreaders 218.sub.1 and 219.sub.1 for multiplying the first code 227 which is identical to the first one of the 20 spreading codes used during transmission by the digital in-phase signal 215 and the digital quadrature signal 216, respectively, to reproduce two transmitting symbols 220.sub.1 and 221.sub.1, a decoder 222.sub.1 for decoding the two reproduced transmitting symbols to a bit signal 223.sub.1, and a frame decomposer 224.sub.1 for deriving information concerning the kind and the number of the spreading codes from a frame format contained in the bit signal 223.sub.1 and delivering receiving data 225 of the first code. The remaining demodulating units 217.sub.2 to 217.sub.20 are constructed similarly to the demodulating unit 217.sub.1.
Next, the operation of a transmitter of the transmitting/receiving apparatus will be described. In a frame assembler 229, transmitting data 228 is arranged in a frame format in accordance with the number of spreading codes used and a data amount of the transmitting data 228. In a transmission code generator 236, spreading codes 237 to be used are determined on the basis of information concerning the spreading codes and spreading code information owned by the base station which are sent from each of the demodulating units 217.sub.1 to 217.sub.20. Transmitting information pieces 230 delivered out of the frame assembler 229 are inputted to 20 modulating units 231.sub.1 to 231.sub.20. The modulating unit 231.sub.1 has a modulator 232.sub.1 for mapping the transmitting information to an in-phase signal (I-ch) 233.sub.1 and a quadrature signal (Q-ch) 234.sub.1, and spreaders 235.sub.1 and 235.sub.2 for spread-processing the in-phase signal 233.sub.1 and the quadrature signal 234.sub.1 by using one of the spreading codes 237. The remaining modulating units 231.sub.2 to 231.sub.20 are constructed similarly to the modulating unit 231.sub.1. Output signals 238 and output signals 239 from the modulating units 231.sub.1 to 231.sub.20 are added by adders 240 and 241, respectively. Output signals 242 and 243 of the adders 240 and 241 are converted from digital signals to analog signals by means of digital/analog converters (D/A converters) 244 and 245, respectively. Output signals 246 and 247 of the D/A converters 244 and 245 are inputted to a quadrature modulator 252 via LPF's 248 and 249, respectively, so that a base-band signal is converted into an IF signal 253. The LPF's 248 and 249 are adapted to eliminate higher harmonic components in the output signals 246 and 247 of the D/A converters 244 and 245 and to shape waveforms of the output signals 246 and 247 of the D/A converters 244 and 245. The IF signal is up-converted from the IF frequency to a carrier frequency by means of an up-converter 254 so as to be converted into a transmitting signal 255. The transmitting signal 255 is inputted to the antenna 201 via the switch 202 and transmitted to the mobile station.
In the aforementioned transmitting/receiving apparatus, however, transmitting data pieces are transmitted while being superimposed on each other by using a plurality of spreading codes and as compared to a transmitting/receiving apparatus adapted to transmit transmitting data by using a single spreading code, the service area is disadvantageously narrowed for the following reasons:
(1) Much transmitting power is required for transmission over the same distance; and
(2) The transmitting distance is reduced when the transmitting power is made to be equal for the both types of apparatuses.